The invention relates to a glass-ceramic which can be employed as a dielectric in the high-frequency range (frequency>200 MHz), more particularly in the gigahertz range (frequency f>1 GHz).
For a series of applications in the high-frequency range, specific materials are needed which combine an extremely high relative permittivity ∈ with an extremely low dielectric loss (tan δ). In order on the one hand to allow small antenna constructions and on the other to prevent close-range detuning by the body of a user (referred to as “body loading”), particular significance attaches to dielectric charging in the case of antennas, filters, and other devices. Required for this purpose are dielectrics which have a high relative permittivity, with ∈≧15, and also a low dielectric loss (tan δ) of not more than 10−2, preferably lower, in the high-frequency range. Furthermore, the temperature dependence of the resonance frequency τf is to be extremely low. Lastly, a material of this kind is to be able to be processed extremely simply and inexpensively in order to allow near net shapes at favorable cost.
Known in the prior art are a series of ceramic materials which are processed by sintering operations. Glass-ceramics are likewise known—cf., e.g., a BiNbO4 system, which is disclosed in Mirsaneh et al., “Circularly Loaded Dielectric-Loaded Antennas: Current Technology and Future Challenges”, Adv. Funct. Materials 18, (2008), pp. 1-8, for application with dielectrically charged antennas for the gigahertz range. This material can be utilized for producing the three principally utilized forms of antennas: the circularly polarized DLA helix antenna (D-LQH antenna) and the square patch antenna, and also SMD antennas. For this purpose, a glass with a composition of 30 mol % Bi2O3, mol % Nb2O5, 30 mol % B2O3, and 10 mol % SiO2 is first melted conventionally at 1250° C. for two hours.
This glass was poured into cylindrical molds, relaxed at 500 to 520° C., and cooled slowly to room temperature. This was then followed by crystallization at various temperatures between 600° C. and 1000° C. Specified as an optimum value for antenna applications in the case of heat treatment at 960° C. is a relative permittivity ∈ of 15, with a quality factor Q·f0 of 15 000 GHz and a temperature coefficient of the resonance frequency τf of −80 MK−1. The crystalline phase characterized in this case was substantially orthorhombic BiNbO4.
This system, using bismuth and niobium is expensive in terms of the raw materials.
In addition there are a series of sintered ceramic materials (cf. U.S. Pat. No. 6,184,845 B1, US 2007/063902 A1). Specified therein as dielectric material for the ceramic core of a dielectrically charged D-LQH antenna is a sintered ceramic material based on zirconium titanate and, respectively, based on zirconium tin titanate, with a relative permittivity of about 36. The material is said to be produced by extrusion or pressing and subsequent sintering.
Further sintered materials are specified in the review by M. T. Sebastian et al., “Low loss dielectric materials for LTCC applications”, International Materials Reviews, Vol. 53, 2008, pp. 57-90. Although these materials are in some cases identified as “glass-ceramics”, they are in fact sintered materials, since they are produced by the sintering of a mixture of vitreous and crystalline powders.
US 2002/0037804 A1 and US 2004/0009863 A1 further disclose dielectric ceramics which are said to form diverse crystal phases, such as, for instance, CaTiO3, SrTiO3, Ba Ti4O9, La2Ti2O7, Nd2Ti2O, Ba2Ti9O20, Mg2TiO4, Mg2SiO4, Zn2TiO4, etc., which are said to be responsible for high quality factors. These as well are sintered ceramics.
Dielectrics produced by sintering have a series of disadvantages: Every sintering operation is always associated with a certain shrinkage, leading to geometrical inaccuracies and corresponding final machining. Furthermore, every sintering operation produces a certain residual porosity, which is disadvantageous in the context of metalizing of the surface. The metal penetrates the pores and raises the dielectric loss of the dielectric.
Moreover, the production of sintered materials is fundamentally relatively inconvenient and expensive.
JP 2006124201 A, moreover, discloses a lead-free glass which is said to be used for producing a dielectric for a printed circuit, having a high dielectric constant and a low electrical loss. The glass contains (in mol %): 25 to 45 SiO2, 5 to 25 BaO, 18 to 35 TiO2, 1 to 10 Al2O3, 0 to 15 B2O3, 0 to 15 MgO+CaO+SrO, 0 to 7 WO+ZrO2, with ZnO<1. It is said to crystallize on heat treatment as BaTi4O9.
JP 2011-195440 A, which corresponds to German patent application DE 10 2010 012 524.5, discloses, in addition, a glass-ceramic comprising the following constituents (in mol % on oxide basis):
SiO2 5-50
Al2O3 0-20
B2O3 0-25
BaO 0-25
TiO2 10-60
RE2O3 5-35
where Ba may be replaced in part by Sr, Ca, Mg, where RE is lanthanum, another lanthanoid, or yttrium, and where Ti may be replaced in part by Zr, Hf, Y, Nb, V, Ta.
This glass-ceramic can be used to produce high-quality dielectrics, which are suitable in particular for high-frequency applications, such as antennas. It has nevertheless emerged as being disadvantageous here that this glass-ceramic as well is not optimized for the production of antennas, which necessitates subsequent metalizing of the surface. The residual porosity is still relatively high for this. Moreover, the raw materials costs of the known glass-ceramic with RE-Ti system and Nb—Ti systems are fairly high.